It’s all going to per mile road charging when all the non compliant vehicles stop paying up , so factor this into your thinking
This will depend on the factors that will be considered when determining the charge.
E.g.:
Distance covered
Location (busy roads will cost more per mile)
Time-of-day (driving during rush-hour will cost will more per mile)
Real-time Congestion (charging based on real-time congestion)
Type of vehicle (different charges for private cars, LGV, HGV, etc)
Exhaust emissions (different charges per mile based on NOx, Particulates, CO2, etc etc etc)
Driver (possible concessions depending on location of residence or other personal circumstances)
So it's not necessarily guaranteed that older Diesel cars will pay the same as all other cars when (and if) PAYG road pricing is introduced - again, it will depend on the factors that will be chosen for use in the formula, and the weight each one is given.